CODEWITHSUNDEEP
javajava-8main-method
Cuban coffee
Cuban coffee

Reputation: 294

Main Method with int, Object and String [] args

I got the below code from internet and even though there are 3 main methods, if I run the app from command prompt:

java MainTest 1 2 3

I would get the output:

String main 1

public class MainTest {

    public static void main(int [] args) {
        System.out.println("int main " + args[0]);
    }
    public static void main(Object[] args) {
        System.out.println("Object main " + args[0]);
    }

    public static void main(String[] args) {
        System.out.println("String main " + args[0]);
    }

}

Thanks a lot!!

Upvotes: 0

Views: 3409

Answers (6)

Holger
Holger

Reputation: 298599

The name main is not reserved by the Java Programming Language. It’s an ordinary identifier and thus, the same rules as with all other identifiers apply. You can have multiple methods named main as long as they have different parameter types. That’s called method overloading.

The special meaning of a certain main method is defined by the Launcher which initializes your application and this startup procedure defines the rules. Note that there might be different, implementation specific, ways to launch an application, e.g. Applets don’t have a main method and JavaFX provides its own Launcher that works without a main method. The same applies to several server-side Java technologies. This doesn’t prevent you from declaring main methods when using such technologies, but they don’t have a special meaning.

So the well-known java launcher, using command line arguments, searches for a special method named main having the raw signature public static void main(java.lang.String[]) in the specified main class. Note that there were earlier Java versions whose launcher didn’t care about the presence or absence of the public modifier. All that matters to the launcher is, whether it finds such a main method. It doesn’t care about other methods of the same name and different signature. It also doesn’t care whether other classes have an appropriate main method. All it does, is searching for an appropriate method in the specified class.

Since only the raw (bytecode level) signature of the method matters, it is irrelevant, which source code form it has, as long as it got compiled to a method with the appropriate signature. Thus, the following declarations will lead to a main method, appropriate for launching a Java application:

public static void main(String[] arg) {}
public static void main(String arg[]) {} // source code variantion don’t matter
public static void main(String... arg) {} // varargs are compiled to an array type
public static final void main(String[] arg) {} // final is irrelevant

public interface Example {
    static void main(String[] arg) {} // public is implied (Java 8)
}

public static <T extends String> void main(T[] arg) {} // raw parameter type is String[]

Note that with the last variant, some IDEs failed to launch the application (despite it works from command line), showing that the actual behavior dependents on the implementation of the launcher.

In contrast, the following doesn’t work:

class String{}
public static void main(String[] arg) // the parameter type is not java.lang.String[]

demonstrating, that not the source code form but the compiled raw signature matters. When trying to run the last example, the launcher will complain that it doesn’t find the required method public static void main(String[]), not telling you that there is a different, non-matching main method. As said, it doesn’t care about other methods of that name.

Upvotes: 1

Ankur Singhal
Ankur Singhal

Reputation: 26077

JVM will only recognize below as a main method.

public static void main(String[] args){} will be used when your class is launched by the JVM.

Also public static void main(String args[]) {} can be used by JVM.

public static void main(String[] args) {
        System.out.println("String main " + args[0]);
    }

rest are overloaded methods

Also we can use a varargs signature, as that's equivalent from a JVM standard

public static void main(String... args)

The positions of public and static may change.

From JavaDocs

Upvotes: 1

Edwin Buck
Edwin Buck

Reputation: 70999

You can write as many static main methods as you like, but the JVM is looking for a specific main method signature.

public static void main(String[] args)

Any other main method will exist (and can be called by some other Java element) but those other main methods will not be called by the JVM at startup time.

Upvotes: 0

mayank agrawal
mayank agrawal

Reputation: 658

you are defining simple methods, JVM will not call them directly except public static void main(String[] args) . so it's upto your requirement in which you need a method with overloading concept and it's static so it can be called directly using the class name.

as per my understanding, it's up to your requirement only.

Upvotes: 0

Gaurav Jeswani
Gaurav Jeswani

Reputation: 4592

In java the method which is having exactly below signature :

public static void main(String[] args)

Having special rights and considered the Main method. Other are just overloaded methods.

Upvotes: 0

SpringLearner
SpringLearner

Reputation: 13854

The example shows overloading

main method is overloaded.

JVM only looks for main method which is static and it should accept only one argument of String array. So you are getting String main as output

Is there any scenario where more than 1 main method is needed in the same class?

It only depends on your requirement

I have not used yet the main method with int, Object [] args? is that even possible? I thought it was only possible with String[] args

I think you want to call the main method with Object[] argument. public static void main(Object[] args) { is like any other simple method and you can call it by MainTest.main(new Object{1,2})

I was expecting getting an error message at runtime. Why I did not get any error when the application ran? Does it mean that the parameters passed are always parsed to String and Java gets the main method

The code is perfectly fine and so you are not getting any error.The code describes an example of overloading of main method

Upvotes: 2

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